Browsing by Author "Levecque, Pieter"

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    CdSe based nanowires for the photocatalytic production of hydrogen gas
    (2018) Abdullah, Ilyaas; Levecque, Pieter; Blumenthal, Mark
    Photocatalytic production of hydrogen was investigated towards achieving a decarbonized supply of hydrogen gas for clean energy conversion technologies such as the proton exchange membrane fuel cell (PEMFC). This study uses a template-directed electrodeposition technique to synthesize multi-segmented CdSe based nanowires for use as a photocatalyst device for hydrogen production. CdSe, Ni, Au and Pt nanowires were successfully synthesized with dimensions ranging from 100 nm to 350 nm in diameter and up to 10 µm long. The CdSe stoichiometry was not easily controlled despite following literature protocols and requires a more systematic investigation. The electrodeposition of Ni nanowires was found to be most effective with very few problems encountered. Improvements in the morphology of Au and Pt nanowires were made by using a constant current as opposed to constant potential electrodeposition techniques. Multi-segmented nanowire devices were prepared with nanowires left embedded in a porous anodized aluminium oxide (AAO) template. Polymer PEDOT: PSS and noble metal Pt was used as an anode and cathode electrocatalyst materials respectively. A prototype photocatalytic testing system was set-up using a 1600 W xenon arc lamp as a light source, an in-house made photoreactor as the device holder, and a mass spectrometer for online gas detection measuring ionic currents of evolved species. The set-up was able to successfully detect hydrogen evolved during the tests but does require further development if more complete photocatalytic testing is to be conducted in future. Photocatalytic hydrogen production from the irradiated devices was inconclusive, but hydrogen detection from devices was observed in an 80 % MeOH solution with no irradiation. Through these tests it was learned that photocatalytic activity needs to be differentiated from regular catalytic activity. This is particularly the case if testing is conducted in organic media and if the photocatalytic phenomena is to be properly isolated and understood correctly
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    CuAg bimetallic nanoparticles for the electrochemical reduction of carbon dioxide
    (2021) Dlamini, Gcinisizwe; Kooyman, Patricia; Levecque, Pieter
    The electrochemical reduction of carbon dioxide is a surface reaction, involving the conversion of carbon dioxide and water to hydrocarbons and oxygenates in an electrolytic environment. This reaction grants an opportunity for the rerouting of carbon dioxide from expulsion to the atmosphere towards the production of chemical products. Due to the stable C-O bond in carbon dioxide, this reaction requires a catalyst and an external energy source to activate it. The use of renewable energy as an energy source would ensure that the electrochemical reduction process is carbon neutral. Cu has been identified as a promising catalyst for the electrochemical reduction of carbon dioxide, as it is more active and produces higher amounts of hydrocarbons and oxygenates relative to other transition metals,. However, Cu is unselective towards a specific product, and it highly active for the undesirable hydrogen evolution reaction (Kuhl et al., 2014). On the other hand, under electrochemical conditions, Ag yields mainly CO, which has been shown to compete with the hydrogen evolution reaction (Hori, Murata & Takahashi, 1989). This study focuses on the synthesis of different ratios of CuAg bimetallic nanoparticles, and their electrocatalytic performance evaluation for the electrochemical reduction of carbon dioxide. Bimetallic nanoparticles were synthesised via a wet chemical method using two synthesis routes. One synthesis was performed in the presence of hexadecylamine (HDA), surfactant, while the other was performed in its absence. The electrocatalytic performance evaluation was conducted using two reactors, a batch reactor with a gas diffusion electrode, and a rotating disc electrode reactor. It was found that catalysts synthesised in the absence of HDA had a phase-separated atomic arrangement, forming islands of Cu and Ag. On the other hand, synthesis conducted in the presence of HDA culminated in a CuAg solid solution. The two synthesis routes resulted in catalysts that had distinct product distributions. Catalysts prepared in the absence of HDA predominantly formed formate, with catalysts that had a higher Cu content forming methanol and CO. The yield of formate for catalysts synthesised under the absence of HDA did not decline at higher potentials relative to Cu catalysts which suffered from hydrogen production. On the other hand, bimetallic catalysts synthesised in the presence of HDA demonstrated behaviour similar to monometallic catalysts. Catalysts with a higher Cu content predominantly produced formate, while catalysts with a high Ag content produced a CO rich stream. This study indicates a profound dependency of the catalyst activity and product distribution on the CuAg bimetallic ratio and atomic arrangement. This study adds knowledge on the synthesis of CuAg bimetallic nanoparticles, and the design of catalysts for the electrochemical reduction of carbon dioxide.
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    Design, construction and commissioning of an automated optical fibre catalyst coating process for use in photocatalytic reactor systems
    (2020) Harrisankar, Naomi; van Steen, Eric; Levecque, Pieter
    Climate change is one of the greatest challenges facing humanity. Fossil fuels are the primary source of energy on Earth. Since the global economic growth is closely linked to the global energy demand, fossil fuel usage remains the largest contributor to the steadily increasing atmospheric carbon dioxide concentration (CO2). CO2 mitigation through carbon capture and conversion are of great interest. Capturing CO2 from point source emitters is possible by absorption in a basic, sodium hydroxide (NaOH) containing solution, which is then converted into sodium bicarbonate (NaHCO3). Conversion of CO2 is thermodynamically demanding as it will require a large amount of energy, which renders currently used technologies infeasible. A promising alternative is the conversion of captured NaHCO3 into useful hydrocarbons at moderate operating conditions using solar energy, by a process called photocatalysis. Photocatalysis is the acceleration of a photo-induced reaction in the presence of a catalyst. Photocatalytic reactors have not yet been commercialised due to suboptimal catalyst and reactor designs. The typically low catalyst activity has to be countered by efficiently loading a large amount of catalyst in the reactor. This results in a problem regarding the photon transfer limitations to the catalytically active site, which limits illumination of the catalyst in the reactor. This can be overcome by using optical fibre to guide photons, which are coated with the photocatalyst. However, it is estimated that a reactor containing ca. 1 g of catalyst will require ca. 1.8 km of identically coated optical fibre. The aim of the project is to design, construct and commission an automated controllable process to increase the production volume of catalyst coated optical fibre using either a solgel suspension or a slurry containing P25 (TiO2). A multi-step optical fibre coating process was developed to achieve the desired coated optical fibre as a product. It consists of 6 major units that process raw (polymer-coated) optical fibre into catalyst coated optical fibre. The steps include the 4 essential steps required for optical fibre preparation by-hand, these steps are stripping, washing, coating and heat treatment. This automated optical fibre catalyst coating process (AOFCCP) can make the coating of optical fibres time-efficient and controllable. The latter can be achieved by controlling the effect various process parameters affecting the coating thickness and homogeneity of the coating, such as pH, heat treatment, catalyst slurry concentration as well as pulling speed. The AOFCCP produced coating thicknesses ranging from 0.47 µm - 0.59 µm and 0.37 µm - 0.46 µm for the P25 slurry and sol-gel coating methods respectively. The pH of the P25 slurry was found to have a negligible effect on both the coating thickness and surface morphology, therefore is no longer regarded as a process variable in the AOFCCP. The thickness of the coating increased with an increase in P25 slurry concentration with a maximum achievable coating thickness of 0.87 µm using a slurry concentration of 20 wt.-%. The temperature of heat treatment which was tested showed different relationships between the coating methods. For the sol-gel coating method, the increase in temperature resulted in a decrease in coating thickness possibly due to the decrease in porosity whereas for the P25 slurry method the increase in temperature showed an increase in coating thickness possibly due to the higher evaporation rates. An increase in the pulling speed in the AOFCCP resulted in an increase in coating thickness on the optical fibre independent of the coating method; coating thicknesses ranging from 0.41 µm - 0.71 µm and 0.23 µm - 2.14 µm were obtained using the P25 slurry and sol-gel coating methods, respectively, by varying the pulling speed. The critical cracking thickness is defined as the thickness of the film, produced by the sol-gel method, at which coating deformations become observable which was found to be 0.37 µm at 600 °C, and 0.77 µm at a pulling speed of 2.30 mm.s -1 . The results obtained from the commissioning experiments showed that the AOFCCP can produce coated optical fibre with controllable thickness. The controllability was discovered to be in the adjustment of the process variables investigated which showed a significant effect on the coating thickness, except for pH. Based on the statistical analysis that was performed, it was confirmed that the results obtained from the system were repeatable and that the coating was uniform for all process variables that were investigated except for sol-gel coating at high speeds of 2.88 mm.s -1 – 3.46 mm.s -1 . The system was able to produce fibre with coating thickness's between 0.4 – 1.1 µm. It is recommended that a combination of the process variables be used in order to achieve better controllability in the process and to achieve thicker coating layers. Furthermore, the operating ranges of the process variables should be increased in order to determine the extent of the relationship between the process variable and the coating thickness and surface morphology.
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    DFT Study of MAX Phase Surfaces for Electrocatalyst Support Materials in Hydrogen Fuel Cells
    (2020-12-25) Gertzen, Jonathan; Levecque, Pieter; Rampai, Tokoloho; van Heerden, Tracey
    In moving towards a greener global energy supply, hydrogen fuel cells are expected to play an increasingly significant role. New catalyst support materials are being sought with increased durability. MAX phases show promise as support materials due to their unique properties. The layered structure gives rise to various potential (001) surfaces. DFT is used to determine the most stable (001) surface terminations of Ti2AlC, Ti3AlC2 and Ti3SiC2. The electrical resistivities calculated using BoltzTraP2 show good agreement with the experimental values, with resistivities of 0.460 µΩ m for Ti2AlC, 0.370 µΩ m for Ti3AlC2 and 0.268 µΩ m for Ti3SiC2. Surfaces with Al or Si at the surface and the corresponding Ti surface show the lowest cleavage energy of the different (001) surfaces. MAX phases could therefore be used as electrocatalyst support materials, with Ti3SiC2 showing the greatest potential.
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    MAX phases as an electrocatalyst support material: a DFT study
    (2019) Gertzen, Jonathan; Rampai, Tokoloho; Van Heerden, Tracey; Levecque, Pieter
    The insatiable global demand for energy cannot be sustained by fossil fuels without irreparable damage to the environment. Various alternative energy sources are being investigated to provide renewable clean energy. One promising technology is the hydrogen fuel cell, which uses hydrogen and oxygen to produce electricity. However, the currently used catalyst support material, carbon black, corrodes in the low pH and oxidative environment. Therefore, new catalyst support materials are being sought. A new class of material, called MAX phases, shows potential because some possess a combination of properties of metals and ceramics. Three of them, Ti2AlC, Ti3AlC2, and Ti3SiC2, show good electrical conductivity and oxidation resistance. These MAX phases have been investigated using density functional theory (DFT) in this thesis to determine their properties. The density of states show that they are electrically conductive, with a continuous band over the Fermi level primarily from the Ti d orbital. Calculating the Boltzmann transport properties, yielded electrical resistivity values of 0.460 µΩ m for Ti2AlC, 0.370 µΩ m for Ti3AlC2, and 0.268 µΩ m for Ti3SiC2 at 300 K. Therefore, Ti3SiC2 should be the most electrically conductive of the three. The vacancy formation energy of an A group atom was investigated using a 2 x 2 x 2 supercell. The vacancy formation energies were calculated to be 2.882 eV for Ti2AlC, 2.812 eV for Ti3AlC2, and 2.167 eV for Ti3SiC2. The formation of a vacancy increases the electrical resistivity of the bulk MAX phases. As a catalyst support material, a MAX phase particle will have surfaces present. Due to the layered structure of the MAX phases, multiple terminations of (0 0 0 1) surfaces could be possible, which were investigated. It was shown that terminations where the Ti-C cage structure remained intact produced the lowest cleavage energies. For Ti2AlC, the two low cleavage energy surfaces are Al(Ti) and Ti(C), for Ti3AlC2, Al(Ti2) and Ti2(C), and for Ti3SiC2, Si(Ti2) and Ti2(C). The surfaces with the lowest cleavage energy should be more stable than other surfaces and would therefore be expected to be present on a MAX phase particle. Vacancies were also formed in the surface systems. The surfaces with the vacancy in the surface layer had the lowest vacancy formation energy, with that of Si(Ti2) being positive. The surface slabs generally showed a higher electrical resistivity than the bulk systems, while the formation of a vacancy generally increased the resistivity, in agreement with the bulk vacancy trend. These MAX phases are electrically conductive, however a quantifiable oxidation resistance was not able to be calculated. They do however show signs of being good electrocatalyst support materials.
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    Pulsatile Electropolishing of Nitinol Stents
    (2019) Cloete, Jeran Andre; Bezuidenhout, Deon; Levecque, Pieter
    Alloys that oxidize easily such as those containing titanium or chromium present a challenge to electropolishing because the polarization that dissolves the metal species produces positive ions, these oxidize and form stable surface layers of metallic oxides that prevent further dissolution. This is usually overcome with the use of acid solutions that dissolve the metallic oxide. This thesis aims to shift the primary control of the electropolishing e_ect from electrolyte variables to a combination of potential variation and hydrodynamic interference. Traditionally this is achieved with one continuous mass removal process that operates after a steady state of dissolution is established, generally requiring hydro_uoric or phosphoric acid to achieve titanium dioxide breakdown. The resulting concentration gradient is heavily a_ected by electrolyte variables such as viscosity and electrical resistance, while the electrical polarization is constrained by the metallic oxide reaction rate which creates a complex net of interdependent variables that can be di_cult to tune. A rapidly changing electric _eld was applied to modulate the alloying element dissolution rates. In tandem with the electropolishing development, stages prior to the electropolishing step were selectively removed to simplify the process. Utilizing a three electrode system and an external potentiostat controller to permit greater _exibility, a variety of alternating current pulsatile waveforms were investigated and the resulting e_ect on surface topology was observed using SEM and AFM microscopes. Di_erential pulse voltammogram yielded a feedback parameter on surface composition, and various pulse parameters were adjusted to optimize for surface smoothness, and identify the primary control variable. An electropolishing method is presented which achieves a :50% reduction in the Sa surface roughness value to an area average of 45 nm on a laser cut tubular stent geometry. It is shown that this method can be adapted to eliminate the need for chemical etching or mechanical polishing prior to electropolishing. The resulting polished surface displays corrosion resistance equivalent or better than other electropolished Nitinol surfaces from literature with a breakdown potential >1V vs SCE, and a similarly high repassivation potential. Balancing the charge in the anodic and cathodic pulses was the key to minimizing the resulting surface roughness, and eliminating micropits. Nitinol is a nearly binary alloy of NiTi and a charge transfer ratio of 1 yielded the smoothest surfaces at current densities around :1 A/cm2. The initial surface condition was found to be irrelevant to electropolishing control with respect to oxide composition, provided enough mass was removed to fully dissolve the initial layers of mixed composition.
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    Ti3C2Tx as an Advanced Support Material for Polymer Electrolyte Fuel Cell Catalysts to Facilitate the Oxygen Reduction Reaction
    (2022) America, Tyler Daniel; Rampai, Tokoloho; Levecque, Pieter
    Polymer electrolyte fuel cell's (PEFC's) have the potential to offer a leading energy conversion technology. These fuel cells make use of hydrogen and oxygen and by means of a chemical reaction, electricity, heat and liquid water are produced. In 2015 the Department of Energy (DoE) of the United States declared a 5000-hour lifetime target for transport applicable fuel cells. With current technological limitation, the achieved lifespan is, however, restricted to only 1700-hours. An assessment to find a more active but primarily more durable support for the oxygen reduction reaction (ORR) in a PEFC than the currently employed carbonaceous support was therefore undertaken. A MXene, Ti3AlC2 was selected for assessment based on its theoretically suitable electrical and thermal conductivities, as well as its possession of among the strongest resistance to oxidation of the many different MAX phases. The synthesis of high (>50 m2 g -1 ), specific surface area, delaminated Ti3C2Tx flakes was attempted first with mild in-situ HF conditions. While this method could both etch and delaminate flakes in a single stage, because the flake size remained large and unchanged, the specific surface area was not seen to increase to the outlined requirements. To synthesize Ti3C2Tx flakes with a high specific surface area, HF etching was therefore employed. In this report, 0.5 g of 400-mesh Ti3AlC2 flakes synthesized by hot pressing were etched in 10 ml of 48 wt % HF for 24 hours at 30 °C. After micronizing for 10 minutes and probe sonicating in solution for a further 40 minutes, high specific surface area (86 m2 g -1 ), delaminated Ti3C2Tx flakes were attained. Using metal organic chemical deposition, well dispersed 2- 5 nm platinum particles were successfully deposited onto the support material. Initial electrochemical performance evaluations indicated a lack of conductivity which restricted electron transport and therefore limited catalyst activity. This was determined to be the result of more defective flakes and by correlation, an increase in interfaces leading to increased resistance. With the incorporation of carbon to the catalyst material to synthesize a hybrid electrode, a positive result confirming ORR activity was attained. While the electrochemical surface area (ECAS) was less than half of that of Pt/C (80 vs 28 m2 g -1 ), it confirmed where the synthesis constraints lie. In review of the durability results, it was found that trapped intermediates between high specific surface area MXene sheets not only restricts access to catalytic sites but are further protonated and reduced to form hydrogen peroxide which causes irreversible damage the PEFC's catalyst membrane.
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    Towards identifying platinum anchor sites on carbon via a model electrochemical system
    (2018) Fortuin, Adrian Charles; Levecque, Pieter; Scherer, Günther
    The interaction between Pt and its carbon support was investigated by a model electrochemical system. This entailed aggressively oxidising a two-dimensional carbon substrate, i.e. highly orientated pyrolytic graphite (HOPG) and mirror finish graphite (MFG) quartz crystal, to incorporate oxygen terminated groups into the graphitic matrix. This study focusses on potential cycling to determine the mobility of Pt across these carbon surfaces and the effect of the Pt anchoring to carbon on the electrocatalyst durability. This work incorporates both a conventional three electrode electrochemical setup and the use of the electrochemical quartz crystal nano-balance (EQCN). The objectives of this study were to better understand the Pt mobility across the carbon substrate surface and to gain insight into the solid-liquid interface of Pt dissolution due to potential cycling. Initial results on HOPG as discussed in chapter 2, indicated minimal Pt dissolution of between 13% and 15% of total electrochemical active surface area loss. These results, however, did not provide adequate evidence to conclusively determine the extent of Pt mobility on the carbon surface and the effect of oxygen terminated groups in hindering Pt dissolution. In order to gain a more thorough understanding of the Pt dissolution processes, the use of the EQCN technique was utilised. Firstly, it was shown that the mirror finished graphite quartz crystals used in the EQCN technique, are qualitatively comparable to the electrochemical measurements recorded with the HOPG samples. Secondly, potential cycling under the same conditions as HOPG produced similar electrochemical results. The frequency response curves from the EQCN yielded the most promising results. This study showed, qualitatively, that the surface of Pt is non-monotonic, and that the surface charge changes with increased potential cycling. Pt/MFG-A had consistent frequency responses over the entire potential range during Pt dissolution, thus, with the above understanding of surface charge, it is concluded that acid treated carbon substrates show a stronger affinity for Pt anchoring.